1. Field of the Invention
The present invention relates to a method for manufacturing a paste for electroconductive thick film. More specifically, the present invention relates to a method for manufacturing a paste for electroconductive thick film used for an internal electrode of a laminated ceramic electronic part, a paste for electroconductive thick film obtained by a method according to the present invention, and a laminated ceramic electronic part using it.
2. Description of the Related Art
A paste for electroconductive thick film, such as a paste for electroconductive thick film for screen printing for use in a laminated ceramic electronic part, is usually prepared by kneading an electroconductive component comprising a metallic powder such as Ni, Cu, Ag or Pd with an organic vehicle prepared by dissolving an organic resin component such as ethyl cellulose or an acrylic resin into a relatively high-boiling point main solvent such as carbitol or terpineol, followed by dispersion using a dispersion machine. The paste for electroconductive thick film for use in screen printing should be adjusted to have a viscosity of 10 Pa.s or more to attain a high printing precision. Three rolls are generally employed as a means for dispersing such a highly viscous paste.
When a metallic powder included in the solid component is a fine powder having an average primary particle size of 1.0 xcexcm or less in the above described method for manufacturing a paste for electroconductive thick film, the aggregation power between powder particles is increased due to the increased specific surface area of the solid component powder, so that dispersion with three rolls or the like cannot achieve full disintegration of the aggregated solid component powder, thus resulting in the problem of undispersed solid component aggregate remaining in the paste for electroconductive thick film.
There is a method in which a dispersant additive is added to a mill base before the dispersion treatment in order to achieve a solid component containing a powder having an average primary particle size of 1.0 xcexcm or less dispersed uniformly in a paste. However, in the case of a dispersion method using three rolls or the like, it was found difficult to have a surfactant, such as a dispersant, uniformly adsorbed on the surface of a solid component powder since the viscosity of the mill base comprising a solid component and an organic vehicle was high. Particularly, those polymeric dispersants which are adsorbed on the functional group of a surface of a solid component powder in a multiple point configuration to prevent aggregation of particles by stearic hindrance-repulsion and electrostatic repulsion between the particles, are mostly viscous liquids at ordinary temperature. It was, therefore, difficult to have them adsorbed uniformly on the surface of the solid component powder. Also, there was the problem that an organic resin component constituting the organic vehicle prevented the dispersant from being adsorbed on the surface of a solid component powder.
There is another method to attain a high dispersion effect by mixing an organic resin component with a dispersant after having it adsorbed on a powder surface. However, it was difficult to obtain a mill base having a viscosity suitable for the three-roll dispersion method or the like, by using a composition composed only of a solid component, a main solvent and a dispersant. Thus, it was unavoidable to add an organic resin component to the mill base before the dispersion treatment, and as the organic resin component prevented the dispersant from being adsorbed, a paste for electroconductive thick film having an excellent dispersibility of a solid component could not be obtained.
If a paste for electroconductive thick film having such undispersed particles of a solid component is used as a paste for screen printing of an internal electrode of a laminated ceramic electronic part, the undispersed particles of the solid component will appear as protruded portions on the dried coated film and penetrate the ceramic layer, thus causing a short circuit defect.
To overcome the above described problems, preferred embodiments of the present invention provide a method for manufacturing a paste for electroconductive thick film and a paste for electroconductive thick film having little undispersed particles of a solid component in the paste and excellent in dispersibility, even when the solid component of the paste for electroconductive thick film contains a powder having an average primary particle size of 1.0 xcexcm or less, as well as a laminated ceramic electronic part with only a small defective rate due to short circuit.
One preferred embodiment of the present invention provides a method for manufacturing a paste for electroconductive thick film using a solid component, a diluting solvent, a dispersant, an organic resin component and a main solvent, wherein the boiling point of the above-mentioned diluting solvent is lower than that of the above-mentioned main solvent by 100xc2x0 C. or more, and the above-mentioned diluting solvent is compatible with the above-mentioned organic resin component and the above-mentioned main solvent, the method comprising a first dispersion step for obtaining a first slurry by subjecting a first mill base prepared through mixing the above-mentioned solid component, the above-mentioned diluting solvent and the above-mentioned dispersant to a dispersion treatment, a second dispersion step for obtaining a second slurry by subjecting a second mill base prepared through mixing the above-mentioned first slurry with the above-mentioned organic resin component and the above-mentioned main solvent to a dispersion treatment, and a step for removing the above-mentioned diluting solvent from the above-mentioned second slurry.
The above-mentioned first mill base may further comprise the main solvent.
The above-mentioned first mill base may further comprise the organic resin component, the amount of said organic resin component contained in said first mill base being less than or equal to one-third of the total amount of the organic resin component contained in the paste for electroconductive thick film.
The above-mentioned step for removing the diluting solvent may further comprise an evaporation-removal process using at least either one of the methods of heating and reducing pressure.
The above-mentioned dispersant may be a fatty acid or salt. The fatty acid or salt may be either stearic acid, oleic acid or a metallic salt thereof. The fatty acid may be added to a paste for electroconductive thick film in an amount of about 0.05-5.0 parts by weight.
The above-mentioned dispersant may be an anionic dispersant. The anionic dispersant may be a polymer having a weight average molecular weight of about 5,000 or above. The anionic dispersant may comprise a monomer having at least one group one selected from a carboxylic acid, a sulfonic acid, a phosphoric acid and a neutral salt thereof. The anionic dispersant may be added to the paste for forming a thick film in an amount of about 0.05-10.0 parts by weight.
The above-mentioned solid component may contain at least a metallic powder. The solid component may contain at least a powder having an average primary particle size of about 1.0 xcexcm or less.
The above-mentioned paste for electroconductive thick film may be a paste for screen printing.
Another preferred embodiment of the present invention provides a paste obtainable by the method of the present invention.
Another preferred embodiment of the present invention provides a laminated ceramic electronic part characterized by an internal electrode formed by using a paste for electroconductive thick film obtainable by the manufacturing method of the present invention.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.